1. Field of the Invention
The present invention relates to a reading lens, more specifically, to a lens, a lens system and an image scanning apparatus including such lens and provided in a manuscript reading unit of a facsimile machine, a digital copier, various image scanners or similar devices.
2. Description of Related Art
Although conventional manuscript reading units provided in a facsimile machine, a digital copier, an image scanner or similar devices has a reading density of 200-400 dots per inch ("dpi"), recent demand for greatly improved image quality requires that such a manuscript reading unit have a higher reading density. It is expected that an increase in reading density would provide a desired amount of improvement of image resolution, a reduction of moire and other image-related improvements.
In currently available manuscript reading units provided in a facsimile machine, a digital copier or similar device, a minimum size of a pixel of a CCD (Charge Coupled Device) is about 7 .mu.m. In such a manuscript reading unit, a reading lens requires a resolution and a contrast for a field spatial frequency as high as 71.4 lines/mm. Furthermore, when an attempt to increase the reading density is made without changing the size of the pixel of the CCD, it is necessary to increase an imaging magnification. For example, although the imaging magnification is 0.11024 when the reading density is 400 dpi, the increase of the reading density to 600 dpi results in the imaging magnification being increased to 0.16535. Although the varying imaging magnification naturally requires a change in the design of the reading lens, the higher magnification is disadvantageous to a correction of aberration. Moreover, the reading lens is required to have a wide angle of view which is capable of reducing a conjugate length (a distance between an object and an image) to achieve a reduction in size and miniaturization of an apparatus including such reading lens and manuscript reading unit. The reading lens is also required to have a large aperture for high-speed reading. Of course, a low cost for the reading lens and manuscript reading unit are also required. These are common requirements for a reading lens regardless of the reading density.
Since most of the known reading lenses have been designed for a reading density of 400 dpi or less, when such conventional reading lenses are used with a reading density of 600 dpi by changing the imaging magnification, a sufficient imaging performance cannot be achieved.
An example of such a conventional reading lens which is designed so that it may be used with the imaging magnification of about 0.16535 is disclosed in Japanese Patent Application Laid-open No. 2-256013. The reading lens described therein is designed so that it may be used with an imaging magnification of about 0.16535. However, since any glass material for use in this reading lens has a refractive index of 1.75 or more and is therefore relatively expensive, the reading lens described in Japanese Patent Application Laid-open No. 2-256013 cannot provide a low cost reading lens or manuscript reading unit. In addition, this reading lens does not have a desired high level of brightness and angle of view.